The present invention relates to a hologram recording method and a recorded hologram. More particularly, the present invention relates to a method of continuously recording holographic interference fringes in a recording film, and also relates to a recorded hologram.
As a method of recording holographic interference fringes, it is well-known practice to make light incident normally on a recording material. There are, however, cases where a beam of light is made incident on the surface of a recording material at a relatively small angle thereto in order to select various pitches for interference fringes to be recorded. However, as the angle of incidence with respect to the surface of the recording material decreases, the rate at which the incident light is reflected due to the refractive index difference between the air and the recording material rises, resulting in a reduction in the quantity of light entering the recording material. Consequently, the energy of light entering the inside of the hologram dry plate per unit of time decreases, and the exposure time lengthens correspondingly. To cope with the problem experienced when light is made incident on the surface of the recording material at a relatively small angle, the conventional practice has been to allow the light beam to enter the recording material through a transparent member having an incidence surface approximately perpendicular to the incident light. For this purpose, a glass block or a prism is used as a transparent member.
FIG. 17 shows an example in which glass blocks are used. A pair of glass blocks 42 and 43 each having a refractive index close to that of a recording material 41 are brought into close contact with both sides, respectively, of the recording material 41, and light beams 44 and 45 are made incident on the surfaces of the recording material 41 through the glass blocks 42 and 43, thereby eliminating reflection at the surfaces of the recording material 41 and increasing the amount of energy incident per unit of time, and thus enabling the desired hologram to be recorded efficiently.
FIG. 18 shows an example in which prisms are used as transparent members. A pair of prisms 46 and 47 are disposed on both sides, respectively, of the recording material 41 in the same way as in the arrangement shown in FIG. 17 so that light beams 44 and 45 which are incident substantially normally on the respective surfaces of the prisms 46 and 47 interfere with each other in the recording material 41, thereby recording the desired hologram.
There has also been proposed a technique whereby a glass block is rotated so that the incidence surface of the glass block is always perpendicular to the incident light beam, although not shown in the accompanying drawings, (see Japanese Patent Application Laid-Open (KOKAI) No. 3-237481).
In addition, Japanese Patent Application Laid-Open (KOKAI) Nos. 1-154079 and 3-271788 disclose a method wherein a plane of a transparent member is brought into close contact with a recording film to expose the film through the transparent member, thereby continuously producing a holographic mirror.
However, the conventional hologram recording method in which transparent members are brought into close contact with both sides of a recording material suffers from the problem that exceedingly large transparent members are needed particularly when exposure is effected over a large area, so that the system becomes costly and bulky and also increases in weight.
The conventional method in which a holographic mirror is continuously produced by effecting exposure through a transparent member is a method for forming interference fringes parallel to the surface of a recording film and not a method for continuously forming interference fringes inclined with respect to the film surface. To reflect sunbeams incident along a direction other than the normal direction as in the case of a heat ray reflecting film in particular, it is preferable to record interference fringes inclined with respect to the film surface because the reflection efficiency can be considerably increased by doing so, as described later.
With the conventional method, a holographic mirror can be recorded in principle, but since the interface of close contact between the recording film and the transparent member used for the incidence of a light beam is a plane, many problems as stated below occur in practice, causing the quality of the produced hologram to be degraded:
Firstly, the recording film may be damaged when contacted by the edge of the transparent member.
Secondly, since the interface between the recording film and the transparent member is a plane, air bubbles inevitably get mixed in the index matching liquid, causing a failure of holographic recording in regions where the air bubbles are present.
Thirdly, since the amount of index matching liquid to be supplied is large due to the structure of the recording system, the index matching liquid is likely to become non-uniform. In addition, the transparent member and the recording film are contaminated with the index matching liquid. Therefore, it is likely that unnecessary interference fringes will occur and the diffraction characteristics will become non-uniform.
Fourthly, since the recorded interference fringes lie in only one direction (parallel to the film surface) and cannot be multiplexed, it is difficult to obtain a holographic mirror providing a wide diffraction spectrum as in the case of a heat ray reflecting film.